PROCESS FOR THE PREPARATION OF A SULFATE- AND HYDROXIDE-BASED FERRIC COMPOUND

Abstract: The invention relates to a process for the preparation of a pure and solid, sulfate- and hydroxide-based ferric compound such as ferric hydroxysulfate or sodium jarosite, the process comprising the following procedures: a) a sulfate- and/or chloride-based ferric salt is prepared, b) an acidic aqueous solution is prepared which comprises either sulfate ions and ferric ions or sulfate, chloride and ferric ions, c) the acidic aqueous solution is maintained for 2-12 h at a temperature of 60-220 DEG C for the production of a solid, sulfate- and hydroxide-based ferric compound, such as ferric hydroxysulfate or sodium jarosite, by hydrolysis, and d) the pure and solid sulfate- and hydroxide-based ferric compound, such as ferric hydroxysulfate or sodium jarosite, is separated from the acidic aqueous solution. The processes for the preparation of ferric hydroxysulfate and sodium jarosite differ substantially only in that in the preparation of sodium jarosite an acid aqueous solution is prepared which, in addition to the either sulfate and ferric ions or sulfate, chloride and ferric ions, also contains a water-soluble sodium compound, in which case sodium jarosite is formed instead of ferric hydroxysulfate in the hydrolysis of point c) of the process.

Description: Process for the preparation of a sulfate- ant- hydroxide-based ferric compound. The invention relates to a process for the preparation of a pure and solid, sulfate- and hydroxide-based ferric compound. According to the method it is possible to prepare from impure raw materials, such as ferrous sulfate or ferric hydroxide, a pure, solid and useful iron compound. Iron is one of the major impurities in waste waters. This is due to a plurality of factors, such as the high reactivity and abundance of iron. Iron is present in conjunction with almost all other metals in their ores. Iron also reacts readily with most acids and forms aqueous or water-soluble iron salts. The principal sources of the iron in waste waters are the waste solutions from steel pickling, zinc electrolysis, titanium pigment preparation, mining, extraction of non-iron metals, metal coating of iron, and copper extraction. Examples of iron removal methods used in industry include various complicated regeneration methods used for steel pickling solutions, evaporation combined with the crystalliza¬ tion of iron salts, transportation deep into the ground, dis- posal in storage basins, disposal by dumping in the sea, and disposal by neutralization. There is thus in the art a clear need for effective and inexpensive removal of iron from me¬ tallurgical processes and for its effective reuse. Iron salts have been used for the treatment of waste water for more than 80 years. The low solubility of trivalent iron, i.e. ferric iron, within a wide pH range is a clear advantage over aluminum salts. Within the pH range (pH 2-10) important in water and waste water treatment, the salts of bivalent iron, i.e. ferrous iron, are slightly soluble, for which reason the ferrous iron content of the ferric salts to be used in water purification should be minimized for the purpose of obtaining pure water. Normally the water treatment chemical, such as ferric sulfate, is prepared by dissolving the raw material in sulfuric acid and by oxidizing Fe(2+) to Fe(3+). Ferric sulfate may be prepared either from ferrous sulfate or from iron oxides or hydroxides. Ferrous sulfate for its part is prepared by dissolving iron in sulfuric acid. Commercial ferric sulfate has been prepared conventionally by oxidizing ferrous sulfate by means of either hydrogen peroxide or nitric acid. Depending on the molar ratio of iron to sulfate, either monomeric ferric sulfate or polymeric polyferric sulfate is thereby formed. The commercially produced ferric or polyferric sulfate with its impurities remains in an aqueous solution, and no satis- factory purification step has so far been found for the pro¬ cesses. Therefore the heavy metals present in the raw material have also been left in the end product of the process. En¬ vironmental legislation becoming stricter, water treatment and other such chemicals which do not contain toxic heavy metals will be needed in the future, and thus the processes so far used for the preparation of ferric and ferrous sulfate will require workable and economically profitable purification steps. GB-1 469 193 discloses a process for continuous production of ferric hydroxysulfate by heating ferrous sulfate (FeS0 4 x 7 H 2 0) with hot, oxygen rich gases the temperature of which is within the range 46-480 °C in a reaction zone into which a portion of the end product is recycled so that the zone will receive a primary material which contains recycled end product in an amount of only 10-25 % by weight. The substantially dry feed is heated in a long reaction zone which rotates about its central axis and is divided into a plurality of oblong chambers, electric heating generating a temperature which is at maximum 480 °C, and preferably air heated to a temperature of 250-300 °C is fed into the chambers. This state-of-the-art system is very expensive and is not as such usable for the preparation of iron hydroxysulfate. Furthermore, the pro- cess lacks the capacity to separate from the initial material, i.e. iron sulfate, the impurities present in it, such as toxic heavy metals. An object of the invention is to provide a new process for the preparation of a pure and useful iron chemical from impure raw materials. It is also an object of the invention to refine the ferrous sulfate produced as a byproduct of inorganic process¬ es, such as the titanium dioxide process, into a useful iron compound. At the same time it is an object to solve the en¬ vironmental problem caused by ferrous sulfate, pickling sol¬ utions based on hydrochloric acid or sulfuric acid, and iron hydroxide precipitates. More precisely, the invention aims at the preparation of a pure ferric compound, usable as a water treatment chemical, based on sulfate and hydroxide. The objects of the invention stated above have now been achieved with a process of a new type for the preparation of a ferric compound based on sulfate and hydroxide, the process being mainly characterized by the facts stated in the charac¬ terizing clause of Claim 1. It has thus been realized that a pure and solid ferric compound based on sulfate and hydroxide can be prepared by the following procedure: a) a sulfate- and/or chloride-based ferric salt soluble in an acid aqueous solution is prepared al) by recovering a sulfate- and/or chloride-based ferric salt produced as a product of an industrial process, soluble in an acid aqueous solution, a2) by oxidizing ferrous sulfate FeS0 4 in a sulfuric acid solution, a3) by sulfating ferric hydroxide Fe(OH) 3 with sulfuric acid, a4) by chlorosulfating ferric hydroxide Fe(0H) 3 with sul¬ furic acid and hydrochloric acid, a5) by chlorinating ferric hydroxide Fe(0H) 3 with hydro¬ chloric acid, or a6) by oxidizing ferrous chloride FeCl 2 in a hydrochloric acid solution, and b) an acid aqueous solution of a sulfate- and/or chloride- based ferric salt is prepared, c) the acid aqueous solution is maintained for 2-12 h at a temperature of 60-220 °C in the presence of sulfate ions, in order to produce a precipitate of the said sulfate- and hydroxide-containing ferric compound by hydrolysis, and d) the sulfate- and hydroxide-containing ferric compound is separated in a pure and solid form from the acid aqueous sol¬ ution. In the process according to the invention, the initial ma¬ terial used is a sulfate- and/or chloride-based ferric salt, such as ferric sulfate and/or polyferric sulfate and/or ferric chlorosulfate and/or ferric chloride, or a mixture thereof. These initial materials may be used in the form in which they • are obtained from various industrial processes, but according to one preferred embodiment they are obtained by oxidizing in point a) above a sulfate- and/or chloride-based ferrous salt. According to one embodiment, the ferrous salt of point a) is ferrous sulfate, in which case the sulfate-based ferric salt, such as ferric sulfate and/or polyferric sulfate, is prepared by causing ferrous sulfate, sulfuric acid and an oxidant to react by contacting them with each other in an aqueous soluti- on at an elevated temperature. Preferably they are contacted with each other at a temperature of 80-160 °C and a pressure of 3-10 bar, most preferably at a temperature of 120-130 °C and a pressure of 5-7 bar. Under these conditions, a sulfate- based ferric salt, i.e. ferric sulfate, is formed. The said oxidation of ferrous sulfate to ferric sulfate is deemed to occur mainly in accordance with the following reaction equ¬ ation (I) : 4FeS0 4 + 2H 2 S0 4 + 0 2 = 2Fe 2 (S0 4 ) 3 + 2 H 2 0 (I) When ferrous sulfate is oxidized in the presence of sulfuric acid and an oxidant, it is advantageous to use the said re¬ agents so that the molar ratio S0 4 /Fe(II) is approximately 1.10 - 1.50. If it is desired to oxidize the ferrous sulfate to monomeric ferric sulfate, it is preferable to use such amounts of sulfuric acid and ferrous sulfate that the molar ratio S0 4 /Fe will be in the order of 1.40 - 1.50. If, instead, it is desired to prepare polyferric sulfate, preferably the amounts of sulfuric acid and ferrous sulfate should be such that the molar ratio S0 4 /Fe will be in the order of 1.10 - 1.40. As stated above, the raw material of the process according to the invention may also be a chloride-based ferric salt. It is hydrolyzed in point c) of the process in the presence of sul¬ fate ions into a ferric compound based on sulfate and hydrox¬ ide. According to one embodiment, the said chloride-based fer¬ ric salt is obtained by oxidizing a chloride-based ferrous salt, such as ferrous chloride. Points a) - c) of the process according to the invention may be carried out separately in succession, or advantageously they may be carried out all in one step. In the latter case, the process may start with an aqueous solution to which the raw material is added and on which the said procedures are suitably performed by changing the reaction conditions and by adding reagents. It is also possible to add all the reagents immediately at the beginning and to perform in one single heating step the preparation and hydrolysis of the said ferric salts. If the ferric salt based on sulfate and/or chloride is prepared by oxidation, and the solution obtained from the oxi¬ dation is used as such for the hydrolysis of point c) , it is advantageous to optimize the concentrations of the ions par- ticipating in the reactions, such as the sulfate, chloride and ferric ions, with respect to both the formation of ferric salt and hydrolysis. The idea of the invention is based exactly on this possibility. According to one embodiment, the oxidation of point a) and the hydrolysis of point c) are combined by causing the ferrous sulfate, or the ferrous sulfate and ferrous chloride, to react with water and an oxidant. The reaction will occur, for example, during heating for 2-12 h at a temperature of 60- 220°C. According to one preferred embodiment, a temperature of 80-160 °C is used, most preferably a temperature of 120- 130 °C. In this case the pressure may be raised to 3-10 bar, preferably to 5-7 bar. In a ferrous sulfate oxidation hydro¬ lysis such as this, ferric hydroxysulfate is formed mainly according to the following reaction formula (II) : 4FeS0 4 + 2H 2 0 + 0 2 = 4Fe(0H)S0 4 (II) When, according to one embodiment of the invention, the fer¬ rous sulfate and ferrous chloride are introduced into point a) in the form of a pickling solution based on sulfuric acid or hydrochloric acid, the oxidation of point a) and the hydroly- sis of point c) may be combined by allowing the pickling sol¬ ution based on hydrochloric acid or sulfuric acid to react, at an elevated temperature, with an oxidant. The heating is carried out preferably to a temperature of approximately 80- 160 °C, most preferably to a temperature of 120-130 °C, in which case a preferred pressure is approximately 3-10 bar, most preferably approximately 5-7 bar. In this case the oxi¬ dation of the ferrous sulfate to ferric hydroxysulfate and the oxidation of the ferrous chloride to ferric hydroxychloride take place mainly according to the following reaction equations (III and IV) : 4FeS0 4 + 2H 2 0 + 0 2 = 4Fe(0H)S0 4 (III) 4FeCl 2 + 2H 2 0 + 0 2 = 4Fe(OH)Cl 2 (IV) A situation has been discussed above in which a sulfate- and/- or chloride-based ferric salt has either been taken directly from an industrial process or been produced by oxidation. The oxidant used may be any oxidant that oxidizes inorganic salts, such as oxygen, nitric acid, chlorine, hydrogen peroxide, sodium chlorate and/or air. Oxygen has proven to be an es¬ pecially usable oxidant in the process according to the inven¬ tion. The oxidation itself may be carried out in either one or a plurality of steps in order to obtain an optimal oxidation result. According to another embodiment of the invention, differing from those described above, a sulfate- and/or chloride-based ferric salt is produced in point a) of the process by reacting ferric hydroxide with sulfuric acid and/or hydro-chloric acid. In this case, according to one embodiment, the ferric hydroxi¬ de and the hydrochloric and/or sulfuric acid are contacted in an aqueous solution and are heated preferably to a temperature of 40-140 °C, most preferably to a temperature of 60-100 °C. Under these conditions the ferric hydroxide neutralizes and dissolves to ferric sulfate and/or ferric chloride mainly according to the following reaction equations (V, VI and VII) : 2Fe(OH) 3 + 3H 2 S0 4 = Fe 2 (S0 4 ) 3 + 6H 2 0 (V) Fe(0H) 3 + 3HC1 = FeCl 3 + 3 H 2 0 (VI) 3Fe(OH) 3 + 2H 2 S0 4 + 5HC1 = FeCl 3 + Fe 2 (S0 4 ) 2 Cl 2 + 9H 2 0 (VII) In point b) of the process according to the invention, an acidic aqueous solution is produced which comprises either sulfate and ferric ions, or sulfate, chloride and ferric ions. On the basis of the foregoing it is clear that the solution for the preparation of raw materials in point a) can be used as such, provided that the solution is suitable for the hydro¬ lysis of step c) . A second alternative is to modify the prep¬ aration solution of step a) , for example, by concentration, dilution, acidification with for example sulfuric acid, or neutralization. Regardless of how the acidic aqueous solution of point b) is produced, it has been found preferable for the molar ratio S0 4 /Fe(III) to be in the order of 0.6 - 1.7. When necessary, the sulfate concentration of the acidic aque¬ ous solution of point b) can be adjusted by adding a sulfate salt such as magnesium sulfate and/or sodium sulfate. When a chloride-based ferric salt is present in the acidic aqueous solution of point b) , the sulfate content must be so high that a substantial proportion of the chloride-based ferric salt is converted in hydrolysis to ferric hydroxysulfate or its de¬ rivative. In general, the composition of the acid aqueous solution of point b) may vary greatly depending on whether the process according to the invention is used for preparing fer¬ ric hydroxysulfate or some other sulfate- and hydroxide-based ferric compound, such as sodium jarosite. The main idea is, however, that the sulfate- and/or chloride-based ferric salt hydrolyzes to a solid hydroxysulfate compound of trivalent iron, whereupon the impurities present in the initial ma¬ terials are left in the acidic aqueous solution and the product is recovered in pure form. One of the most important advantages of the invention is that in principle hardly any sulfuric acid needs to be added to the hydrolysis step. Ferric salts are as such acid salts in an aqueous solution, one idea of the invention being that only a certain heating for a certain time is required for the hydrolysis. In point c) of the process according to the invention, the hydrolysis of the acidic aqueous solution of point b) , which contains either sulfate and ferric ions, or sulfate, chloride and ferric ions, is carried out by heating, whereupon a sul¬ fate- and hydroxide-based ferric salt is precipitated. The hydrolysis time is 2-12 h and the temperature is 60-220 °C. These conditions are thus suitable for producing both ferric hydroxysulfate and various types of sulfate- and hydroxide- containing ferric compounds by hydrolysis. In the hydrolysis it is preferable to heat for 3-8 h, in which case the pressure may be, for example, 3-15 bar, preferably 5-10 bar. Up to now the discussion has been of those characteristics of the process according to the invention which are common to the preparation of all the sulfate- and hydroxide-based ferric compounds referred to in the invention. Below, the special features of those embodiments of the invention are discussed by which ferric hydroxysulfate on one hand and sodium jarosite on the other hand are prepared. These are the most important compounds prepared by the process according to the invention. According to one embodiment of the invention, the procedures of points a) - c) are performed without an alkali metal, in which case a pure and solid ferric hydroxysulfate is recovered in point d) . In the production of ferric hydroxysulfate by the process according to the invention it has been found prefer¬ able to adjust the final molar ratio S0 4 /Fe(III) in the acidic aqueous solution of step b) to approximately 1.34-1.70, more preferably approximately 1.34-1.50, and most preferably approximately 1.34-1.40. It should be pointed out that the ratio 1.34 corresponds to a situation in which the initial material of the hydrolysis step b) is a mere aqueous solution of polyferric sulfate. One of the greatest advantages of the ferric hydroxysulfate embodiment of the invention is that in principle no sulfuric acid needs to be added to the hydrolysis step. Since ferric sulfate is as such an acid salt which in an aqueous solution produces a pH value of 1.75, the pH of the acidic aqueous sol¬ ution of point b) of the process according to the invention is preferably within the range 1.14-2.00, and preferably 1.70- 1.80, in which case it is preferably maintained at that value during the hydrolysis of point c) of the process. If sulfuric acid is present in the hydrolysis of point c) , its concen¬ tration will not exceed 60 g/1 and is preferably below ap¬ proximately 30 g/1. As already stated, the temperature in the hydrolysis step c) is within the range 60-220 °C. A preferred hydrolysis tempera¬ ture for point c) of the ferric hydroxysulfate embodiment of the invention is 170-200 °C, and the most preferred tempera¬ ture is 170-180 °C. The respective preferable reaction time of this embodiment is 5-8 h and the most preferable reaction time is 6-7 h. One very important viewpoint of the invention is that the hy¬ drolysis conditions according to the invention may be applied to the initial solution, i.e. the acidic aqueous solution of point b) , which contains very different amounts of ferric iron. Considering commercial production it is, of course, pre¬ ferable to use maximally high iron amounts, in which case the consumption of other chemicals and energy, in proportion to the amount of usable sulfate- and hydroxide-based ferric com¬ pound produced, will be much lower. When the process according to the invention is used for the production of pure ferric hydroxysulfate, the iron concentration of the acidic aqueous solution of point b) can be adjusted to 10-200 g/1, preferably approximately 30-200 g/1, and most preferably approximately 100-200 g/1. An especially advantageous iron concentration is approximately 180 g/1. In the foregoing there have been presented all those charac¬ teristics of the process according to the invention which are common in the preparation of all sulfate- and hydroxide-based ferric compounds and those characteristics which are advan- tageous in particular in the preparation of ferric hydroxysul¬ fate. Even though the last-mentioned characteristics are es¬ pecially advantageous in the preparation of ferric hydroxysul¬ fate, they can, of course, also be used in the preparation of other sulfate- and hydroxide-based ferric compounds. According to another important embodiment of the present in¬ vention, a sulfate- and hydroxide-based ferric compound, so¬ dium jarosite, is prepared, having approximately the formula NaFe 3 (OH) 6 (S0 4 ) 2 , but this may vary, cf. examples. In the pro- cess there is produced in point b) an acidic aqueous solution which, in addition to the above-mentioned sulfate, ferric and possibly chloride ions, also comprises a water-soluble sodium compound, in which case in point c) there is formed through hydrolysis a solid sodium jarosite free of heavy metals etc. In this case, according to one embodiment, the acidic aqueous solution of point b) may be neutralized partly with a base before the heating in point c) . Most suitable for this purpose is a sodium-containing base, e.g. sodium hydroxide or sodium carbonate. In the preparation of sodium jarosite it is also possible to combine the procedures of points a) and b) of the process by producing a sulfate- and/or chloride-based ferric salt in situ in an acidic aqueous solution of a sodium salt. In this case, for example, the oxidation and the hydrolysis may be carried out either simultaneously or as successive steps for the treatment of the same solution. According to one modification of the sodium jarosite embodi¬ ment of the process according to the invention, the acidic aqueous solution of point b) may be produced by dissolving in water a sodium compound selected from among the following: sodium chloride, sodium hydroxide, sodium sulfate, and sodium carbonate. In the sodium jarosite embodiment of the present process, the various components are preferably added so that the molar ratio Na/Fe(III) in the acidic aqueous solution of point b) of the process is adjusted to a value which is in the order of 0.3-1.0. It is also preferable to adjust, by means of a base, the molar ratio OH/Fe(III) in the acidic aqueous sol¬ ution of point b) to a value which is in the order of 0.8-1.3. As stated above in connection with the preparation of ferric hydroxysulfate, one very important aspect of the invention is that the hydrolysis conditions according to the invention may be applied to the initial solution, i.e. the acidic aqueous solution of step b) , which contains varying amounts of ferric iron. The concentration of iron in the acidic aqueous solution of step b) may in the sodium jarosite embodiment be adjusted to a value of 10-200 g/1. When sodium jarosite is prepared by the process according to the invention, the temperature in the hydrolysis step c) is preferably approximately 100-140 °C, the most preferred tem¬ perature being approximately 130-140 °C. A preferred reaction time is approximately 3-6 h, and the most preferred reaction time is 4-5 h. Four figures and 50 examples are presented below to illustrate the invention. Figure 1 depicts variants of the process according to the in¬ vention for the preparation of iron hydroxysulfate (A) and sodium jarosite (B) , Figure 2 depicts a schematic diagram of the iron hydroxysulfa¬ te process, Figure 3 depicts a schematic diagram of the sodium jarosite process, and Figure 4 depicts the final pH of a hydrolysis solution as a function of the hydrolysis time. Figure 2 is a block diagram of the process according to one embodiment of the invention for the preparation of ferric hydroxysulfate by hydrolyzing ferrous sulfate FeS0 4 • 7H 2 0. The ferrous sulfate is fed, together with hydrogen and concen- trated sulfuric acid, into a dissolving unit, from which it is transferred to an oxidation reactor having a pressure of 7 bar and a temperature of 120-130 °C. After the oxidation, pressure hydrolysis takes place at a temperature of 170 °C and a press¬ ure of 7 bar. The hydrolysis time is in this case approxi- mately 5-7 h. Subsequently the ferric hydroxysulfate precipi¬ tated in the hydrolysis is washed and filtered. A washing liquid is added to the washing step, and for example a press¬ ure filter can be used. The obtained product is stored, and the remaining reaction liquid is neutralized with calcium carbonate, and its metallic and other impurities are precipi¬ tated with hydrogen sulfide. The gypsum and sulfide precipi¬ tates from the neutralization and sulfidization respectively are separated, and the remaining water is returned to the process or directed to a drain. Figure 3 depicts schematically a process for the preparation of sodium jarosite by hydrolyzing ferrous sulfate FeS0 4 • 2H 2 0 in accordance with one embodiment of the invention. The fer¬ rous sulfate, together with water, sodium chloride and sodium sulfate, is fed into a dissolving unit, from which it is di¬ rected to an oxidation reactor having a pressure of 7 bar and a temperature of 120-130 °C. The hydrolysis time is approxi¬ mately 5 h. Thereafter the precipitated sodium jarosite is washed and filtered. Washing liquid is added to the washing step, and for example a pressure filter is used in that step. The precipitated product is stored, and the remaining washing liquid is neutralized with calcium carbonate, and the impurity metals are precipitated with hydrogen sulfide. The gypsum and the precipitated sulfides are separated, and the remaining water is returned to the process or directed to a drain. Examples 1-16 (preparation of ferric hydroxysulfate) The experiments were performed in an enameled 2-liter auto¬ clave equipped with a stirrer. The autoclave was capable of being both heated and cooled, and it was equipped with a pressure meter. In the examples, the solution was fed into the autoclave and the autoclave was closed. Most of the examples were performed by using polyferric sulfate as the initial material. When fer¬ rous sulfate was used as the initial material, the oxygen re- quired for the oxidation was fed in as small bubbles to the bottom of the reaction vessel. The oxidation was carried out at a temperature of 120-130 °C, and after the oxidation the autoclave was heated to the desired pressure and temperature for hydrolysis. The heating times of the reaction solution are not included in the oxidation and hydrolysis times reported. After the experiment, the precipitate was separated from the solution and was analyzed for iron and sulfur. The precipitate was identified by means of X-ray diffraction. The solution was analyzed for iron, pH value, and sulfuric acid. The results were as follows. Effect of Fe concentration and temperature Ferric hydroxysulfate can be produced even from very strong solutions of ferric sulfate by pressure hydrolysis. Ferric hydroxysulfate precipitated out even from solutions which con¬ tained iron approximately 170 g/1, as is shown in Table l. Table 1. Ex. Fe H j SQ, pH S0 4 /Fe T(°C) P t Yield S Fe g/i (g/i) bar h (%) (%) (%) 1 45 30 1.65 157 8 3 0 - - 2 45 90 1.16 195 14 3 36 21 32 3 90 30 - 157 10 3 0 - - 4 170 100 1.14 1.68 150 3.5 3 0 - - 5 170 100 1.14 1.68 150 7.8 3 0 - - 6 170 100 1.14 1.68 170 6.5 3 36 20 31 7 170 100 1.14 1.68 170 15 3 28 21 31 According to the results, pressure hydrolysis does not produce ferric hydroxysulfate at temperatures below 150 °C. Even an increase of pressure (in Examples 4 and 5) does not help if the temperature is too low, and does not improve the result even when the temperature is within the correct range (Exam¬ ples 6 and 7) . The obtained final product was ferric hydro¬ xysulfate, which was confirmed by chemical analysis and X-ray diffraction. Effect of HoSO., The effect of the adding of sulfuric acid on the precipitate yield and on the composition of the product is shown in Table 2. The experiments were performed by using as the initial ma¬ terial for the hydrolysis a polyferric sulfate in which the S0 4 /Fe ratio was 1.34. Table 2 Ex. Fe Ωβ0 4 pH S0 4 /Fe T(°C) P t Yield S Fe g/i (g/i) (bar) (h) (%) (%) (%) 8 170 100 1.14 1.68 170 6.5 3 36 20 31 9 172 60 1.34 1.55 170 5.8 3 56 21 31 10 179 20 1.62 1.42 170 6.7 3 44 20 32 11 182 ' 0 1.75 1.34 170 7.0 3 45 20 32 Reducing the amount; of sulfuric acid added does not affect the precipitate yield or the quality of the product. The small scatter of the results may be due to practical problems in the experiment. According to X-ray diffraction, the products ob¬ tained were 2FeS0 4 (0H) or Fe 2 S 2 0 9 •H 2 0, or a mixture of thereof. Thus a high concentration of iron and sulfate in the solution prevents the formation of hematite, and a high operating tem¬ perature prevents the formation of jarosite. The formed sulfuric acid amount (at least approx. 0.5 mol/mol product) must be neutralized when the mother liquor is separ¬ ated from the process. The sulfuric acid forming in the hydrolysis reaction is presented as a function of the reaction time in Figure 4. The figure shows, as a function of the hy- • drolysis time, the final pH value of the hydrolysis solution. The iron concentration was 180 g/1, the temperature was 170 °C, the pressure was 7 bar, and no sulfuric acid was added to the ferric sulfate before the hydrolysis. In the figure, a lengthening of the reaction time increased the yield of ferric hydroxysulfate, in which case more sulfuric acid was released into the solution. This is shown as a lowering of the pH value as a function of the hydrolysis time. Effect of reaction time A lengthening of the reaction time increased the amount of precipitate formed in pressure hydrolysis, as shown in Table 3. A high yield (94 %) is also achieved by an elevation of the temperature (Example 16) . Table 3 shows the effects of the reaction time and the reaction temperature on the yield of ferric hydroxysulfate. In the experiment, no sulfuric acid was added to the ferric sulfate initial material. Table 3 Ex. Fe pH S0 4 /Fe T(°C) P t Yield S Fe (g/i) (bar) (h) (%) (%) (%) 12 182 1.75 1.34 170 7.0 1 18 15 34 13 182 1,75 1.34 170 7.0 3 44 20 32 10 14 182 1.75 1.34 170 7.0 5 86 19 32 15 182 1.75 1.34 170 7.0 7 94 19 31 15 16 182 1.75 1.34 180 9.0 5 94 19 32 A high recovery of ferric iron is achieved by lengthening the reaction time. A second alternative is to increase the tem¬ 20 perature. With short reaction times the reaction does not reach an equilibrium, a fact shown in the chemical analysis of the product. One hour's run does not lead to pure ferric hy¬ droxysulfate, a fact which can be concluded from the low sulfur content in the precipitate. 25 Properties of product The ferric hydroxysulfate obtained by the process according to the invention had very low concentrations of heavy metals. Ac¬ cording to the chemical analysis, the metals remained substan¬ 30 tially in the mother liquor, as shown in Table 4. The weight of the initial solution was, in the experiment concerned, 1490 g, and after the hydrolysis 470 g of precipitate and 748 g of filtrate were obtained. The precipitate yield was 86 %. 35 40. Table 4 Element Filtrate Precipitate Pb <0.0009 % <0.002 % Cd 0.000005 % <0.00001 % Cu 0.00029 % <0.0007 % Cr 0.00092 % <0.002 % Co 0.0076 % 0.0022 % Ni 0.0054 % <0.002 % Zn 0.030 % 0.0053 % The concentrations of several substances analyzed from the precipitate were below the detection limits of the analysis method (flame-AAS) . The precipitate contained only approx. 13 % of the original cobalt amount and only approx. 8 % of the original zinc amount. When the precipitate is dissolved and a 12 % iron solution is prepared from it, the metal content in the liquid is diluted to 1/3. The product obtained was soluble in sulfuric acid, but the dissolving was time-consuming. In the first experiments, a solution which contained iron 11.4 % was achieved. The pre¬ cipitate was also soluble in hydrochloric acid, whereby a 15 % solution was achieved. The pH of the solution was 1.43, which is close to the pH value 1.42 of commercial ferric floccu- lants. The concentration of free acid in the 15 % solution was 2. Examples 17-32 (preparation of sodium jarosite) A first set of experiments were performed in an enameled 40- liter autoclave equipped with a stirrer. The autoclave was capable of being both heated and cooled, and it was equipped with a pressure meter. A second set of experiments were performed in three enameled 8000-liter reactors which were coupled to each other to produ¬ ce a continuous reaction line. The reactors were heatable, and they each had a pressure gauge. In the first set of experiments, the solution was fed into the autoclave and the autoclave was closed. When ferrous sulfate was used as the initial material, the required oxygen was fed in as small bubbles. Oxidation was carried out at a tempera- ture of 120-130 °C, and after the oxidation the autoclave was heated to the desired pressure and temperature for hydrolysis. The hydrolysis and oxidation times indicated do not include the times taken by the heating of the reaction solution. After the experiment, the precipitate was separated from the solution, and the iron and impurities were analyzed. The pre¬ cipitate was identified by X-ray diffraction. The following results were obtained. The said impurities were cadmium, chromium, cobalt, copper, lead, manganese, nickel, vanadium, and zinc. In the second set of experiments, the solution was fed con¬ tinuously to the reactor line. Most of the experiments were performed by using various pickling solution mixtures based on hydrochloric acid and sulfuric acid. Experiments were also performed by using ferrous sulfate as the initial material. As in the first set of experiments, in these also the oxygen was fed in as small bubbles. Oxidation was carried out at a temperature of 120-130 °C, and after the oxidation the autoclave was heated to the desired pressure and temperature for hydrolysis. The oxidation and hydrolysis times indicated do not include the times taken by the heating of the reaction solution. After the experiments, the precipitate was separated from the solution, and it was analyzed for iron and impurities. The precipitate was identified by using X-ray diffraction. The solution was analyzed for iron, pH, and impurities. The re¬ sults are given below. Effect of temperature Temperature is an important parameter. Table 5 shows the re¬ sults of Experiments 17-22, in which the temperature ranged from 80-140 °C. Table 5. Results of Experiments 17-22 Exp. Fe Initial T t Yield Final Na salt pH °C h % pH 17 7.0 2.48 100 5 78 0.50 Na 2 S0 4 18 7.0 2.48 120 5 90 0.22 Na 2 S0 4 19 7.0 2.48 140 5 96 0.10 Na 2 S0 4 20 7.3 2.45 80 3 82 1.24 Na 2 C0 3 21 7.4 2.40 100 3 94 1.07 Na 2 C0 3 22 7.4 2.40 120 3 96 1.03 Na 2 C0 3 According to the results, a sodium jarosite yield over 90 % is not obtained in the hydrolysis before the temperature reaches 100 °C. The best yield was obtained within the temperature range 120-140 °C. The yield was better when sodium carbonate was used instead of sodium sulfate, but the pH was higher when sodium carbonate was used. Effect of Na/Fe ratio The effect of the Na/Fe ratio on the yield of the product is shown in Table 6. The experiments were performed by using fer¬ rous sulfate and oxygen as the initial materials for the hyd¬ rolysis. Table 6. Results of experiments 23-28 Exp. Fe:Na T t Final Yield Na salt °C h pH % 23 3:1.5 110 3 0.24 68 NaCl 24 3:2.0 110 3 0.02 71 NaCl 25 3:1.5 110 5 0.20 84 Na 2 S0 4 26 3:2.0 110 5 0.34 90 Na 2 S0 4 27 3:1.5 110 4 0.58 88 NaOH 28 3:2.0 110 4 0.70 92 NaOH The best yield was obtained with an iron/sodium ratio of 3:2.0. This effect was visible with all the sodium salts. Effects of different sodium salts Table 7 shows the effects of the different sodium salts on the product yield. The experiments were performed using ferrous sulfate and oxygen as the initial materials for the hydroly¬ sis. Table 7 Exp. Fe:Na T t Final Yield Na salt °C h pH % 29 3:2 110 5 0.09 75 NaCl 30 3:2 110 5 0.35 89 Na 2 S0 4 31 3:2 110 5 0.7 92 NaOH 32 3:2 110 5 1.0 95 Na 2 C0 3 The best yield was obtained when sodium carbonate was used. The highest pH values were obtained when sodium carbonate or sodium hydroxide was used. This is, of course, a consequence of the hydroxide present in these bases. When sodium sulfate was used, a better yield was obtained than when sodium chloride was used. This is explained by the fact that, when sodium sulfate is used, the sulfate concentration is higher. This effect was examined in the following exper¬ iments. Effect of sulfate concentration The effect of the sulfate concentration on the product yield is shown in Table 8. These experiments were performed by using ferrous sulfate, ferrous chloride and oxygen as the initial materials for the hydrolysis. Table 8. Results of experiments 33-35 Exp. S0 4 :Fe T t Yield °C h % 33 4:3 110 5 91 34 3:3 110 5 75 35 2:3 110 5 58 The highest yield was obtained with an S0 4 :Fe ratio of 4:3. This demonstrates the importance of the sulfate concentration. Effect of temperature on hydrolysis The effect of temperature on the hydrolysis is illustrated in Table 9. Table 9. Results of experiments 36-41 Exp . T t OH/Fe °C h 36 100 5 0 . 56 37 110 4 0 . 58 38 110 5 0 . 68 39 110 6 0 . 88 40 120 5 0 . 80 41 140 5 0 . 92 The OH/Fe ratio increases at high temperatures. When the tem- perature increases from 100 °C to 140 °C, the OH/Fe ratio in¬ creases by 64 %, from 0.56 to 0.92. The most preferred values of the OH/Fe ratio are between 0.80 and 1.0. An OH/Fe ratio this high yields a very low final pH. A mixture formed of sodium jarosite and a solution having a very low pH yields a very low impurity level in the sodium jarosite product. Effect of reaction time Lengthening of the reaction time increased the amount of pre¬ cipitate; this is clearly seen in Table 10. Table 10. Results of Experiments 42-45 Exp. Yield % T Fe:Na lh 2h 3h 4h 5h °C 42 60 71 77 79 84 110 3 :1.5 43 43 79 83 87 94 110 3 :2 44 31 68 82 85 87 110 3 :2 45 55 85 90 92 92 110 3 :2 45 Table 10 shows the effect of the reaction time on the yield of sodium jarosite. A high yield of sodium jarosite is obtained by lengthening the reaction time. Properties of product The sodium jarosite prepared by the process according to the invention contained very small amounts of heavy metals. On the basis of the chemical analysis, the metals substantially re¬ mained in the mother liquor, as can be seen in Table 11, which shows the distribution of the metals. Table 11. Distribution of heavy metals Exp. Pb Co Cr Cu N& Ml Ni V Zn 46 mother liq. 0% 100% 95% 39% 99% 99% 100% 48% 99% product 100% 0% 5% 61% 1% 1% 0% 52% 1% 47 mother liq. 34% 100% 96% 45% 99% 99% 100% 34% 99% product 66% 0% 4% 55% 1% 1% 0% 66% 1% 48 mother liq. 100% 100% 40% 97% 100% 99% 100% 32% 97% product 0% 0% 60% 3% 0% 1% 0% 68% 3% 49 mother liq. 100% 100% 100% 50% 99% 99% 100% 40% 98% product 0% 0% 0% 50% 1% 1% 0% 60% 2% 50 mother liq. 100% 100% 100% 33% 99% 99% 100% 41% 96% product 0% 0% 0% 67% 1% 1% 0% 59% 4% Cobalt, magnesium, manganese, nickel and zinc can be separated from the iron. These metals remain in an amount of 96-100 % in the mother liquor. Lead, chromium and copper are more sensi- tive to changes in the chemical conditions. This sets limi¬ tations on the concentrations of these heavy metals when they are being separated from iron. 32-48 % of the vanadium remains in the mother liquor. Preparation from ferrous sulfate Ferrous sulfate can be used as the initial material in the preparation of sodium jarosite. By the process according to the invention, a very pure iron compound is obtained in which the concentration of heavy metals is low. Table 12 shows the concentrations of heavy metals in the initial material and in the product, and Table 13 gives the same concentrations per one kg of iron. Table 12. Heavy metal concentrations (ppm) in the ferrous sul¬ fate initial material and in the sodium jarosite product. Fe Pb Co Cr Cu N& Mh Ni V Zn Ferrous 18% 1 25 21 2 4030 543 39 9 146 sulfate Sodium 35% <1 <1 <1 1 33 4 <1 8 7 jarosite Table 13. Heavy metal concentrations (ppm) / kg iron Pb Co Cr Cu Mg Mi Ni V Zn Ferrous 6 139 117 11 22389 3017 217 50 811 sulfate Sodium <3 <3 <3 3 94 11 <3 23 20 jarosite With respect to heavy metals, sodium jarosite is thus a much better raw material for the water chemical than is ferrous sulfate. The relative concentrations of heavy metals in sodium jarosite are very low. Composition of sodium πarosite The product of the process according to the invention is a sodium jarosite the structural formula of which is NaFe 3 (OH) 6 (S0 4 ) 2 . It was shown by the experiments that the structural formula varies and may deviate from the above for- mula. The product is a mixture of hydronium jarosite ( (H 3 0)Fe 3 (OH) 6 (S0 4 ) 2 ) and sodium jarosite (NaFe 3 (OH) 6 (S0 4 ) 2 ) , the average composition being Na 0 . 8 (H 3 O) 0 . 2 Fe 3 (0H) 6 (S0 4 ) 2 . This mixture varies within the following limits: Na 0 . 6 (H 3 O) 04 Fe 3 (OH) 6 (S0 4 ) 2 , NaFe 3 (OH) 6 (S0 4 ) 2 .

Claims: Claims 1. A process for the preparation of a sulfate- and hydroxide- containing ferric compound, characterized in that it comprises the following procedures: a) a sulfate- and/or chloride-based ferric salt soluble in an acidic aqueous solution is prepared al) by recovering a sulfate- and/or chloride-based ferric salt obtained as a product of an industrial process, soluble in an acidic aqueous solution, a2) by oxidizing ferrous sulfate FeS0 4 in a sulfuric acid solution, a3) by sulfating ferric hydroxide Fe(OH) 3 with sulfuric acid, a4) by chlorosulfating ferric hydroxide Fe(0H) 3 with sul- furic acid and hydrochloric acid, a5) by chlorinating ferric hydroxide Fe(0H) 3 with hydro¬ chloric acid, or a6) by oxidizing ferrous chloride FeCl 2 in a hydrochloric acid solution, and b) an acidic aqueous solution of a sulfate- and/or chloride- based ferric salt is prepared, c) the acidic aqueous solution is maintained for 2-12 h at a temperature of 60-220 °C in the presence of sulfate ions, in order to produce a precipitate of the said sulfate- and hydroxide-containing ferric compound by hydrolysis, and d) the sulfate- and hydroxide-containing ferric compound is separated in a pure and solid form from the acidic aqueous solution. 2. A process according to Claim 1, characterized in that the sulfate- and/or chloride-based ferric salt is obtained in step a) by oxidizing ferrous sulfate FeS0 4 and/or ferrous chloride FeCl 2 . 3. A process according to Claim 2, characterized in that the ferrous sulfate FeS0 4 is oxidized in point a2) with sulfuric acid and an oxidant by contacting them with each other in an aqueous solution at a temperature of 80-160 °C and a pressure of 3-10 bar, preferably at a temperature of 120-130 °C and a pressure of 5-7 bar, whereby ferric sulfate Fe 2 (S0 4 ) 3 is pro¬ duced. 4. A process according to Claim 3, characterized in that in point a) sulfuric acid and ferrous sulfate FeS0 4 are used in such amounts that the molar ratio S0 4 /Fe(II) will be 1.10-1.50. 5. A process according to Claim 1 or 2, characterized in that the procedures of points a) - c) are performed substantially simultaneously. 6. A process according to Claim 2 or 5, characterized in that the oxidation of point a) and the hydrolysis of point c) are combined by causing the ferrous sulfate, or the ferrous sul¬ fate and ferrous chloride, to react with water and an oxidant for 2-12 h at a temperature of 60-220 °C. 7. A process according to any of Claims 2-6, characterized in that the ferrous sulfate or ferrous chloride is introduced into point a2) or a6) respectively in the form of a pickling acid based on sulfuric acid or hydrochloric acid. 8. A process according to any of Claims 2 - 7. characterized in that the oxidant used in point a2) or a6) is oxygen, nitric acid, chlorine, hydrogen peroxide, sodium chlorate, and/or air, preferably oxygen. 9. A process according to Claim 1, characterized in that the sulfate- and/or chloride-based ferric salt is obtained in points a3) - a5) by reacting ferric hydroxide with sulfuric acid and/or hydrochloric acid. 10. A process according to Claim 9, characterized in that fer- ric hydroxide is reacted in point a4) with sulfuric acid and/or hydrochloric acid by contacting them with each other in an aqueous solution at a temperature of 40-140 °C, preferably at a temperature of 60-100 °C. 11. A process according to any of the above claims, character¬ ized in that in point b) the molar ratio S0 4 /Fe(III) in the acidic aqueous solution is adjusted to a value in the order of 0.6-1.7. 12. A process according to any of the above claims, character¬ ized in that the concentration of sulfate in the acidic aque¬ ous solution of point b) is adjusted by adding magnesium sul¬ fate and/or sodium sulfate. 13. A process according to any of the above claims, character¬ ized in that in point b) magnesium sulfate and/or sodium sul¬ fate is added in such an amount that a substantial proportion of the chloride-based ferric salt is, in the hydrolysis of point c) , transformed into a sulfate- and hydroxide-based fer¬ ric compound. 14. A process according to any of the above claims, character¬ ized in that the pH of the acidic aqueous solution of step b) is adjusted to 1.14-2.00 preferably 1.70-1.80, and is prefer¬ ably maintained at that value throughout the procedure of point c) . 15. A process according to any of the above claims, character- ized in that the heating in step c) is for 3-8 h. 16. A process according to any of the above claims, charac¬ terized in that in point c) the heating is at a pressure of 3- 15 bar, preferably at a pressure of 5-10 bar. 17. A process according to any of the above claims, charac¬ terized in that the procedures of points a) - c) are performed without an alkali metal, in which case a pure and solid ferric hydroxysulfate is recovered from point d) . 18. A process according to Claim 17, characterized in that in point b) the molar ratio S0 4 /Fe(III) is 1.34-1.50, preferably 1.34-1.40. 19. A process according to Claim 17 or 18, characterized in that in point c) the acidic aqueous solution is maintained at a temperature of 170-200 °C, preferably at a temperature of 170-180 °C. 20. A process according to any of Claims 17-19, characterized in that the concentration of iron in the acidic aqueous sol¬ ution of point b) is adjusted to a value of 30-200 g/1, pre¬ ferably to a value of 100-200 g/1. 21. A process according to any of Claims 1-15, characterized in that in point b) an acidic aqueous solution is prepared which, in addition to sulfate, ferric and possibly chloride ions, also comprises a water-soluble sodium compound, in which case in point c) there is formed a pure and solid sulfate- and hydroxide-based ferric compound, which is sodium jarosite. 22. A process according to Claim 21, characterized in that in point b) the acidic aqueous solution is neutralized partly with a base before the heating of point c) . 23. A process according to Claim 22, characterized in that in point b) the neutralization is carried out partly with a base which is sodium hydroxide or sodium carbonate. 24. A process according to any of Claims 21-23, characterized in that the procedures of points a) and b) are combined by preparing a sulfate- and/or chloride-based ferric salt in situ in an acidic aqueous solution of a sodium salt. 25. A process according to any of Claims 21-24, characterized in that the acidic aqueous solution of point b) is obtained by dissolving in water a sodium compound which has been selected from among the following: sodium chloride, sodium hydroxide, sodium sulfate, and sodium carbonate. 26. A process according to any of Claims 21-25, characterized in that in point b) the molar ratio Na/Fe(III) in the acidic aqueous solution is adjusted to a value in the order of 0.3- 1.0. 27. A process according to any of Claims 21-26, characterized in that in point b) the molar ratio OH/Fe(III) of the acidic aqueous solution is adjusted by means of a base to a value in the order of 0.8-1.3. 28. A process according to any of Claims 21-17, characterized in that in point b) the concentration of iron in the acidic aqueous solution is adjusted to 10-200 g/1, calculated as pure iron. 29. A process according to any of Claims 21-28, characterized in that in point c) the heating is to a temperature of 100- 140 °C, preferably to a temperature of 130-140 °C.

Inventors: JAEFVERSTROEM STEFAN, KENAKKALA TIMO, TUOMARLA JUHANI, JAEFVERSTROEM, STEFAN, KENAKKALA, TIMO, TUOMARLA, JUHANI
Applicants: JAEFVERSTROEM STEFAN, KEMIRA OY, KENAKKALA TIMO, TUOMARLA JUHANI, JAEFVERSTROEM, STEFAN, KENAKKALA, TIMO, TUOMARLA, JUHANI
Application year: 1994
Main classification: C01G49
IPC codes: C01G49